Image Loss
Once image quality is lost in JPEG compression, you can never get it back again. Loss in image quality is also cumulative, meaning you lose a little bit more information each time you decompress and compress an image. Each time you open a JPEG and resave it, you degrade the image further. Not only that, you may introduce new artifacts to the image that prevent the second compression from working as efficiently as the first, resulting in higher file sizes.
It is a good idea to hang on to one copy of the original digital image if you anticipate having to make changes, so your final image only goes through the compression process once. You should also start from an original image each time to experiment with different compression levels. Current web graphics tools (Adobe Photoshop/ImageReady and Macromedia Fireworks) make this easy because they always retain the original and allow you to export graphics with your chosen settings.
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